Gas turbine engines such as those used as aircraft propulsion units include turbojet and turbofan types. A turbofan engine includes a fan section, a compression section, a combustion section, a high pressure turbine section and a low pressure turbine section. The fan section is coupled to the low pressure turbine section while the compression section is coupled to the high pressure turbine section. The compression section together with the combustion section and the high pressure turbine section form the core of the engine. The compression section can be a single unit driven by the high pressure turbine or it may be split into an intermediate pressure compressor section followed by a high pressure compressor section driven by an intermediate pressure turbine and a high pressure turbine respectively. The compression sections can include axial compression stages, centrifugal stages or a combination of both.
It has become increasingly desirable to provide improved compressor performance by reducing the amount of air leakage in compressors of turbofan engines including those employing centrifugal stages. The centrifugal compressor typically includes a centrifugal impeller having blades that are enshrouded by a static impeller shroud. Minimizing the clearance between the impeller blades and the impeller shroud optimizes the capacity of the impeller in pressurizing air to the elevated pressures desired for peak engine performance.
The shroud for a centrifugal compressor is typically mounted to an adjacent compressor casing if present, or to a downstream diffuser assembly. The compressor casing or diffuser assembly is mounted to a support frame or other static component. The support frame or casing structure typically forms part of the structural backbone of the engine. This type of shroud mounting can be problematic in that carcass distortions resulting from asymmetric structural loading (e.g., from take-off rotation, maneuver, and landing) are transmitted to the shroud, resulting in a greater than desired impeller tip clearance.
Centrifugal compressors typically employ a radial diffusing section followed by a turning duct or elbow that is a vaned axial or near axial diffusing and de-swirl section. In some applications, the turning duct and vaned axial or near axial diffusing and de-swirl section is replaced with a set of pipe diffuser assemblies. When a rotor support bearing is located aft of the centrifugal compressor assembly, the bearing housing is typically mounted through the radial diffuser section with sump services accommodated within the radial diffuser. On pipe systems, sump services are accommodated within the radial diffuser section or in an area between pipe assemblies.
Diffuser sections are typically joined together with a bolted flange arrangement at an outboard interface point. This bolted flange joint is a major structural interface of the gas turbine engine and may include an aft leg of the casing assembly. Thus, in a typical arrangement, carcass bending loads pass through the bolted flange. As such, heavy g-loads and asymmetric structural loading (including take-off rotation, maneuver and landing) can cause local distortions at the diffuser outboard flange, which can be in turn transferred to the shroud through its mounting point to the diffuser.
Shrouds are typically designed having adequate clearance such that interference does not occur during the most extreme anticipated carcass distortions that can occur during engine operation, due to mechanical loading, thermal loading, component wear, and the like. Distortion that occurs in the shroud during engine operation can increase component clearance, resulting in excess air leakage in the compressor, leading to overall poor engine performance. Reducing the propensity for shroud distortion would be helpful and could improve compressor performance.